The disclosure of Japanese Patent Application No. 2001-049047 filed on Feb. 23, 2001, including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of Invention
This invention relates to a fuel pump for an internal combustion engine.
2. Description of Related Art
Fuel pumps for internal combustion engines have been widely utilized as a system for supplying fuel with high pressure for a direct-injection internal combustion engine in a cylinder. According to this type of fuel pump, fuel is generally transmitted under pressure by a lifting movement of a plunger in a cylinder of a pump. The lifting movement of the plunger is produced corresponding to a movement of a cam.
FIG. 7 illustrates a schematically sectional view of one example of conventional fuel pumps for internal combustion engines. According to a fuel pump 100, fuel is introduced from a fuel supply port 107 and is pressurized by the lifting movement of a plunger 102 in a chamber 101 defined in a cylinder 106 at the center of the pump. The pressurized fuel is then discharged from a fuel discharging port 108. That is, the plunger 102 inserted in the cylinder 106 is provided with a tappet 109 at a bottom portion of the plunger 102. The plunger 102 is normally biased by a spring in a direction of a cam 103.
Corresponding to an initial start of the internal combustion engine (engine), fuel is introduced into the chamber 101 in the cylinder 106. An electromagnetic valve 105 serving as a fuel introducing valve is hence closed. A rotation of the internal combustion engine, i.e. a rotation of a crankshaft, is transmitted to a camshaft 104 via a power transmitting mechanism. The cam 103 then comes into contact with the tappet 109 and is rotatably driven. The cam 103 is formed to have a fixed sectional shape (cam profile) with a few (1 to 3) circular projecting portions, i.e. projections. Therefore, the lifting movement of the plunger 102 is produced when the projections of the cam 103 come into contact with the tappet 109 and push the tappet 109 upward. The volume of the chamber 101 is hence decreased and fuel is pressurized and discharged. The cam 103 is further rotated and the projections of the cam 103 are separated from the tappet 109. The plunger 102 is then returned to the cam 103 by the spring so that the volume of the chamber 101 is increased. In this case, the fuel introducing valve 105 is opened and new fuel is introduced into the chamber 101.
Fuel is transmitted with pressure by repeating the above-described cycle. However, according to the conventional fuel pump for the internal combustion engine, a sufficient amount of discharged fuel could not probably be ensured for obtaining fuel pressure (injection pressure) required by the internal combustion engine especially when the engine is rotated at a low speed, for example when the engine is initially started.
That is, a lift amount by the lifting movement of the plunger is fixed. A frequency of the lifting movements of the plunger, i.e. the number of strokes per unit of time, is determined by the rotation speed of the engine (r.p.m.). Therefore, when the engine is rotated at a low speed, for example when the engine is initially started, an amount of discharged fuel per unit of time is decreased. Further, when the engine is rotated at a low speed, for example when the engine is initially started, a compressing cycle by the plunger requires a long time. Therefore, the amount of fuel leaked from a clearance between the plunger and the cylinder is increased so that an actual amount of discharged fuel per stroke is decreased. Further, a required amount of fuel injected at a cold start is from two to four times as large as the required amount of fuel injected under the vehicle being normally running.
As a result, the conventional fuel pump for the internal combustion engine may have a problem in that a good performance can not obtained at starting because a desirable fuel injection can not be ensured when the engine is initially started.
Considering the above-described problem, according to the invention, an amount of discharged fuel is increased to obtain a required fuel pressure (injection pressure) when an engine is rotated at a low speed, for example when the engine is initially started. Further, a fuel pump for an internal combustion engine is provided for varying the amount of discharged fuel amount to improve the starting performance.
A fuel pump for an internal combustion engine according to one of the embodiments of the invention transmits fuel with pressure by a lifting movement of a plunger that is caused to lift by a movement of a cam connected to a camshaft. The fuel pump for the internal combustion engine is provided with a lift amount changing mechanism that changes a lift amount of the plunger caused by the cam.
Since this type of fuel pump for the internal combustion engine is provided with a lift amount changing mechanism, the lift amount of the plunger caused by the cam can be varied. Therefore, an amount of discharged fuel per stroke of the plunger is changed and is not determined based only upon a rotation speed of the engine. Therefore, a control of the amount of discharged fuel of the pump can be performed as required. Accordingly, required fuel pressure (injection pressure) can be obtained by increasing the amount of discharged fuel even when the engine is rotated at a low speed, for example when the engine is initially started, so that a starting performance can be improved.
A fuel pump for an internal combustion engine of one of the other embodiments of the invention transmits fuel with pressure by a lifting movement of a plunger that is caused to lift by a movement of a cam connected to a camshaft. The fuel pump for the internal combustion engine is provided with a lift number changing mechanism that changes the number of the lifting movements of the plunger that occur per rotation of the internal combustion engine.
Since this type of fuel pump for the internal combustion engine is provided with the lift number changing mechanism, the number of the lifting movements of the plunger that occur per rotation of the internal combustion engine can be varied. Therefore, the amount of discharged fuel per rotation of the internal combustion engine is changed and is not determined only based upon the rotation speed of the engine. Therefore, a control of the amount of discharged fuel of the pump can be performed as required. Accordingly required fuel pressure (injection pressure) can be obtained by increasing the amount of discharged fuel even when the engine is rotated at a low speed, for example when the engine is initially started, to improve a starting performance.
Further, a fuel pump for an internal combustion engine according to one of the other embodiments of the invention transmits fuel with pressure by a lifting movement of a plunger that is caused to lift by a movement of a cam connected to a camshaft. The fuel pump for the internal combustion engine is provided with a speed changing mechanism that changes a rotation speed of the internal combustion engine, transmits the changed rotation speed to the camshaft, and changes a speed change ratio between the rotation speed of the internal combustion engine and a rotation speed of the camshaft.
Since this type of fuel pump for the internal combustion engine is provided with the speed changing mechanism, the number of the lifting movements of the plunger per rotation of the internal combustion engine can be changed by this speed changing mechanism. Therefore, the amount of discharged fuel per rotation of the internal combustion engine is changed and is not determined only based upon the rotation speed of the engine. Accordingly, a control of the amount of discharged fuel of the pump can be performed as required. Therefore, required fuel pressure (injection pressure) can be obtained by increasing the amount of discharged fuel even when the engine is rotated at a low speed, for example when the engine is initially started, to improve a starting performance.